in reply to: who drives the 4-20ma line in HART multi drop network posted by Anirudh on 16 December 2010 at 09:24:58.
The answer is simple: each of the multidropped field devices controls its own loop current at 4mA (or whatever value is chosen by the device designer). (I am assuming transmitters - which are commonly multidropped, not valve positioners - which are not.) Since the devices are connected in parallel (NOT in series), with a single power supply and load resistor, the currents are added together, for example making a total of 16mA for 4 devices, from the power supply and through the load resistor.So each device will only see, and control, its own contribution to the loop current, NOT the total current.
Watch out for too high a total current for intrinsically safe systems, if you try more than about 4 devices. And, of course, total hazardous-side capacitance!